1. Field of the Invention
The invention relates to a high pressure mercury lamp. The invention relates especially to an ultra-high pressure mercury lamp of the short arc type, in which a discharge vessel is filled with greater than or equal to 0.15 mg/mm3 mercury and in which the mercury vapor pressure during operation at least 150 atm.
2. Description of Related Art
In a projector device of a projection type, there is a demand for illumination of images onto a rectangular screen in a uniform manner with adequate color reproduction. Therefore, the light source is a metal halide lamp filled with mercury and a metal halide. Furthermore, recently, smaller and smaller metal halide lamps, as well as more and more often point light sources, have been produced, and lamps with extremely small dimensions between the electrodes have been used in practice.
Instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with greater than or equal to 200 bar (roughly 197 atm), have been recently proposed. The increased mercury vapor pressure in these mercury vapor lamps suppresses the broadening of the arc, and a considerable increase of the light intensity is provided. This extremely high mercury vapor pressure lamp is disclosed, for example, in Japanese patent disclosure document HEI 2-148561, which is a counterpart of U.S. Pat. No. 5,109,181, and in Japanese patent disclosure document HEI 6-52830, which is a counterpart of U.S. Pat. No. 5,497,049.
In such a light source device used in a projector device, with respect to projection of clear images, it is considered very disadvantageous that devitrification of the discharge lamp occurs. On the other hand, recently the use of the DLP® process (Texas Instruments' Digital Light Processor) using DMD (Texas Instruments Digital Micro-mirror Device) has obviated the necessity of using a liquid crystal cell. For this reason, a still smaller projector device is being used more and more often. On the one hand, there is a demand for high light intensity and a high degree of maintenance of the illuminance of a discharge lamp for a projector device, while, on the other hand, according to the reduction in the size of the projector device, there is a demand for reducing the size of the discharge lamp as well. Hence, there is more and more often a demand for more rigorous operating conditions.
Due to the UV light transmission characteristics of silica glass, it is used as the material of the discharge vessel. The alkali metal component in the silica glass has an adverse effect on the discharge lifetime of the lamp. This mechanism of this effect is broadly described as follows:
Normally, in lamp operation, as a result of radiant heat due to the lamp light and Joulean heat which forms between the electrodes, the lamp body reaches a very high temperature. At this high temperature, the degree of motion of the alkali metal ions (i.e., cations) in the glass is large. The alkali metal ions are attracted from the electrode part by the electrical field formed between the lamp electrodes. In doing so, the alkali metal ions adversely affect the bond between the glass and the electrode part, thus reducing the adhesive strength of the glass/electrode interface. As a result, the service life of the lamp is shortened. The alkali metal component of the inner surface part of the glass accelerates the devitrification of the glass surface during lamp operation, and this becomes the cause of reduction of the illuminance.
Before shipping, the lamps are subjected to a test called aging-after-production in order to sort out faulty products. A process of aging is as follows, for example:
A process of two minutes of operation and one minute off is repeated. Afterwards, an uninterrupted operation of 45 minutes takes place. Conventionally, in ultra-high pressure mercury lamps breakage faults form as a result of detachment in the metal foil components of the hermetically sealed portions during this aging period.